Diphasic Effects of Astragalus membranaceus BUNGE (Leguminosae) on Vascular Tone in Rat Thoracic Aorta

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This study was designed to investigate the effects of the aqueous ethanol extract of Astragalus membranaceus BUNGE (Leguminosae) on rat thoracic aorta. Isometric tension was recorded in response to drugs in organ bath. In endothelium-intact aortic rings, A. membranaceus extract induced a significant dose-dependent relaxation of the rings precontracted by phenylephrine, which could be inhibited by preincubation with L-N(ω)-nitro-arginine methyl ester or methylthioninium chloride. In endothelium-denuded ones, the extract could dose-dependently relax the rings contracted by phenylephrine, not by KCl; and it could also attenuate contractile response to phenylephrine, not to caffeine or phorbol-12,13-diacetate in Ca²⁺-free medium; but it failed to affect the CaCl₂-induced enhancement of contractile response to phenylephrine in Ca²⁺-free medium. These results indicate that nitric oxide signaling and Ca²⁺-handling pathway are involved in the A. membranaceus extract-induced vasodilatation.